Against the background of the continually rising energy prices there has been recently more and more interest for heat recovery from liquid slags, actual types of slags for Swedish conditions especially being slags from shale, blast furnace slags and electric arc furnace slags. Thus, in high-temperature vaporization of shale, for instance, there is obtained a liquid slag having a heat content representing about 15% of the total fossil energy content of the shale. Not least for the vaporization process, it is therefore desirable to attempt to make use of the heat content of the slag. In this context one should endeavor to produce steam at such a high pressure that it can be used internally for the operation of compressors for the oxygen plant required, and/or optional compressors for subsequent processes in the further processing of gases up to the end product, e.g. ammonia, methanol, synthetic petrol, synthetic natural gas or synthesising gas for the organic process industry.
However after all, at the internal utilization of the energy content of the slag the recovery of heat from the slag is a subordinate process. This means in turn that the demands for a robust system with high operational reliability are great. Even relatively short shut-downs caused by the heat recovery equipment threatens the economical gains associated with the introduction of this process step. In accordance with the present invention it has been found possible to provide such a robust system having a simple structure and a high operational reliability.
However, apart from recovering as much as possible of the heat content from the slag there have previously also been endeavours to provide a usable product, an example of the prior art being balls of the LECA.RTM. type for insulation purposes, or the recovery of part of the metal content from slags from metallurgical processes by crushing and for instance magnetic separation. For the reasons given above the methods to be found in the literature are based on the condition of accomplishing suitable granulates adapted for special uses.
A Swedish method, the Merotec method, can be mentioned as an example of the prior art in this field, which method is based on the pulverization of a material in a melted, liquid state by means of solid particles, the kinetic energy of which breaks up and finely divides the melted material, whereupon the heat is regained from granulates in a fluidized bed. The method is described in Swedish Patent specification No. 7401822-7 and Swedish Patent Application No. 7809264-0.
A similar method has been developed by Sumitomo Metal Industries Ltd., in Japan, and according to this method the granulation takes place on a rapidly rotating drum, the heat being regained in a fluidized bed combined with a so-called moving bed. The method is disclosed in "Business Japan", March 1982, page 25.
A method utilized in a commercial plant has been developed by Nippon Kokan K. K., the granulation taking place with the aid of compressed air and the slag particles being blown into a chamber, where the heat is transferred to steam via cooling panels. About 40% of the slag heat content is recovered in the steam. This method is described in Mitsubishi Heavy Industries "Technical Revue", June 1981.
Nippon Steel are developing a process based on the same granulation principle with compressed air, but where the major part of the heat is regained in a fluidized bed. A pilot plant for 18 tonnes of slag per hour is to be built, and was expected to be ready by 1983.
It can finally be mentioned that a method has been developed by Kawasaki Steel Corporation, according to which the heat is recovered in a first step by means of radiant heat and in a second step in a cooling shaft while using recirculated air coupled to an exhaust gas boiler. How the agglomeration takes place is not known to us.
Considerable recirculation of slag takes place in the Merotec method, which means an apparatus with large dimensions and great wear. The method also results in wear problems on the cooling tubes in the fluidized bed, which negatively affects the operational costs and availability. Furthermore, relatively large agglomerates are obtained about 5 mm, which requires high air speeds, e.g. 25-30 m/s, for the maintenance of fluidization.
All the described methods also make demands on the physical properties of the slag if the agglomeration is to function. With the strongly acidic slag obtained at high temperature vaporization of e.g. Kvarntorp shale it does not work, since filament-like balls are formed instead of agglomerate with a well defined shape. It would indeed be possible to recover the heat from such balls as well, but then there would be large problems with respect to deposition.
A prerequisite of the known methods is also relatively small agglomerates, which also results in a relatively large specific surface. This means an increased risk of the leaching out of heavy metals during deposition. It would therefore be desirable if slag from a heat recovery plant could be obtained in the form of larger and more compact pieces. This has been found possible by the present invention. The fact that the product is obtained in larger and more compact pieces can also open up new fields of uses for residues of slags of these kinds.